I did pull the tank. Turns out that the filler area was leaking water that entered round the filler cap and percolated down into the tank bed. The flange was rusted in that area, freely leaking fuel. Also, the return fitting was leaking gas. So I emptied the tank, then steamed it for several hours to distill off the gas. It passed the lighted match test so I tried soldering, miserable failure and the vent tube also fell out as I heated the tank to try to get the solder to take. It didn't take at all, just ran around in balls. Turned to the MIG welder and spent most of the day welding and fill testing with water, welding and fill testing etc. Finally, not too bad, there are a couple of pin holes in the welds but I am sick of chasing tiny leaks so I think I might just seal the welds with the epoxy putty and get the car running again.
Hopefully it will be better if not perfect.
Thanks for your suggestions, you were all right!
The steaming worked a dream, just used a domestic steam cleaner.